Figure 3. Different points located in the SFME system for
characterization.
Furthermore, in this study, methyl orange (MO) is used as a probe for
polarity measurements of the SFME system, as it is sensitive to that of
the local micro-environment, giving a red shift on its maximum
absorption wavelength (λmax) in a medium possessing
higher polarity and
vice
versa.31,32 Therefore, the measurement of different
microemulsions at the fixed RP/O value of 8.16 (Line Ⅰ
in Figure 3) was examined by UV-vis spectroscopy (Figure S2A). The
results show that the λmax of MO exhibits a red-shift
(from 416.3 to 421.9 nm) with an increase of water content
(WW), confirming the enhanced polarity of the
microemulsion. At the same time, this linear increase in the
λmax of MO with the increase of WWindicates clearly that there is no change of microemulsion type in the
test area and all the given systems are assigned to O/W microemulsions,
this also validates the nature of the subregions determined above.
Moreover, the polarity of the microemulsion at point c in Figure 3 is
compared with those of common solvents (Figure S2B). It can be found
that the λmax in pure water and n -propanol are
centered at 464.8 and 414.6 nm, respectively, while it peaks at 422.0 nm
in the binary system without octane
at a RP/W value of
4.78. However, when a microemulsion with a RP/W value of
4.78 contains 9.2 wt. % octane (point c in Figure 3), the
λmax of MO shifts to 419.2 nm. Therefore, the
micro-polarity of the as-constructed microemulsion system exists between
those of bulk water and n -propanol.
Moreover, the formation mechanism of different microemulsions was
investigated using FT-IR analysis to obtain information about their
intermolecular and intramolecular interactions.33 The
spectra of microemulsions with increasing WW (Line Ⅰ in
Figure 3) are shown in Figure 4. There is only one strong and broad
adsorption peak in the range of 3000 to 3800 cm-1 for
each microemulsion system, which is attributed to the stretching
vibration of hydroxyl groups. Interestingly, this adsorption peak is
broadened and shifts to lower wave numbers with the increasing
WW (Figure 4A) and it can be contributed to the
formation of hydrogen bonds by the addition of
water.34 In order to clearly demonstrate this effect,
the hydroxyl band was deconvoluted using an appropriate method described
by Gao et al. 35 Three distinct water species
can be identified, i.e. , trapped-, bound- and free-water.
According to the fitted results (Figure 4B), trapped-water (ca.3610 cm-1) exists as a comparatively small fraction in
this system and is considered to be dissolved in octane or bonded with
the alkyl chain of the n -propanol molecule. The bound-water
(ca. 3480 cm-1) is identified as forming
hydrogen bonds with the hydroxyl group of n -propanol. The
absorbance observed at 3348 cm-1 is characterized as
belonging to the O-H stretching vibration of n -propanol, and the
one centered at 3210 cm-1 is assigned to that of free
water due to the formation of strong hydrogen bounds among
themselves.36